py: Fix TracebackDump not to dump duplicated exception type

[corona10]

Fixes: #23

[codecov-io]

Codecov Report

Merging #36 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@ Coverage Diff @@## master #36 +/- ## ========================================== + Coverage 65.23% 65.25% +0.01%  ========================================== Files 56 56 Lines 10151 10148 -3 ========================================== Hits 6622 6622 + Misses 3067 3064 -3  Partials 462 462

Impacted Files	Coverage Δ
py/exception.go	34.54% <0%> (+0.91%)	⬆️

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[corona10]

@ncw
PTAL, proper unit test can't add this time.
Maybe we can add capturing fmt.Fprintf but not the easy job to this.

[ncw]

PTAL, proper unit test can't add this time.
Maybe we can add capturing fmt.Fprintf but not the easy job to this.

Well I think it is a definite improvement, so I'll merge it - thank you :-)